In 1928, the English economist Arthur Pigou coined the term “human capital” to describe investments in the acquisition and application of knowledge, asserting that these investments, just like traditional physical capital, were essential ingredients of economic production. Today, we might divide human capital into three major components. Education plays an important role in developing human capital, of course. Equally important is whether a society’s farms, factories, offices, and shops take advantage of its workers’ abilities through efficient management and the latest technology. And welcoming foreigners infuses a country’s stock of human capital with fresh energy and initiative.

I once asked the boss of Tata Consulting Services, a gigantic Indian IT firm, how many of his top executives had worked or studied abroad. He replied: "All of them."

The world's most talented people are exceptionally mobile. When they move to America, they make it smarter, and that's not just because they are smart. It is also because migration creates connections.